# ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

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"The Primary Rate Interface consists of 23 B-channels and one 64 kbit/s
D-channel using a T1 line. Thus, a Primary Rate Interface user on a T1 line
can have up to 1.544 Mbit/s service."

If you do the math:
23Bchannels x 64 + 1D channel x64 = 1536 kbps or 1.536Mbit/s.
Even if you divide 1536 : 1024 still doesn't give 1.544 Mbps ....

Re: ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

A T1 line has 8000 frames each second of 192 data bits.  When
channelized for voice or ISDN PRI, you have 8 bits for each of the 24
channels in each frame.  Each frame also has a framing bit.  So
193x8000=1.544mb/s, with 8000bps being overhead.  Not unlike a 9600bps
async link really being able to transmit 7680bps of "real" data
(assuming N-8-1 format).

Re: ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

I do not know where you got that, but even so, PRI _uses_ T1 line, and every
book out there says that PRI consists of 23 B-channels _64kbps_each_ and 1 D
channel also _64kbps.

Re: ISDN PRI - 1.544 Mbps or 1.536 Mbps ?
eager wrote:

Hi,

24 * 64kbps  + 8kbps management channel used by telco

Hubert

Re: ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

Right, but what's your question?  Just like I said, each T1 frame of
192 data bits (of which there are 8000 each second), carries eight
bits from each of the 24 (23B+1D) 64kbps channels in a PRI (assuming
the T1 is carrying a PRI, of course).  And each frame has a 193rd bit
for framing, which is the source of the 8000bps you were missing.

The 193x8000 frame format is inherent in a T1.  You can channelize
that many different ways, but with voice-like circuits, the sub
channels are invariably bit sliced across the frame in such a way that
no jitter, burstyness, or lag is introduced.  Remember that basic
voice service is 8000 eight bit samples per second, and that you'll
find line/frame formats all over the (telco) network synchronized in
some fashion to the 8000 samples-per-second rate for the precise
reason that you can carry that data rate without jitter, burstyness,
or lag.

Re: ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

Thank you!